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seminar on introduction to MAC protocols

seminar on introduction to MAC protocols

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  • 1. Comparison of MAC protocols in wired systems and wireless systems Mridula Sharma 19.07.2010 Comparison of MAC protocols in wired systems and wireless systems
  • 2. Outline
    • Introduction
    • Multiple Access Protocols
    • Wired Network
    • Wireless Networks
    • Conclusion
    • References
    Comparison of MAC protocols in wired systems and wireless systems
  • 3. Introduction
    • Large class of networks is built on top of broadcast channels
    • Problem : if you’re sharing a channel, then two stations may decide to start frame transmission at the same time
      • Frame collision , which means rubbish on the wire
    • Solution : Allocate the channel to one of the competing stations
    • Allocation can be of two types:
      • Static: FDM and TDM
      • Dynamic
    Comparison of MAC protocols in wired systems and wireless systems
      • Protocol to determine who goes next on a multi-access channel belongs to the
      • Medium Access Control (MAC) sublayer
        • Especially important for LANs
        • Provides addressing and channel access control mechanisms
  • 4. Introduction Comparison of MAC protocols in wired systems and wireless systems
  • 5. Introduction Functions of MAC layer Comparison of MAC protocols in wired systems and wireless systems
    • On transmission, assemble data into a frame with address and error detection fields
    • On reception, disassemble frame, and perform address recognition and error detection
    • Govern access to a LAN transmission medium
  • 6. Multiple Access Protocols
    • ALOHA
    • Carrier Sense Multiple Access (CSMA) Protocols
    • Collision Free Protocols
    • Limited Contention Protocols
    • Wavelength Division Multiple Access Protocols
    • Wireless LAN Protocols
    Comparison of MAC protocols in wired systems and wireless systems
  • 7. ALOHA
    • Radio-based communication network
      • Developed in 1970s at the University of Hawaii
      • Applicable to any system with competing uncoordinated users
    • Two versions
      • Pure ALOHA
      • Slotted ALOHA
    Comparison of MAC protocols in wired systems and wireless systems
  • 8. Pure ALOHA
    • Let users transmit whenever they have data to be sent !!!
    • If a collision occurs, finish your current transmission and retry later
    • Performance is maximized if all frames have the same size
    • Random waiting time required else collisions occur
    Comparison of MAC protocols in wired systems and wireless systems Fig. : Frames transmitted at completely random intervals
  • 9. Pure ALOHA- Efficiency
    • Probability that k frames are generated during a given frame time with G attempts per packet time:
    • Let P0 be the probability that frame does not suffer from collision
      • Throughput , S= G.P0
    • V ulnerable period = 2Tframe
    • Probability that a frame will not be damaged during two frame times
    • long is:
    Comparison of MAC protocols in wired systems and wireless systems
  • 10. Slotted ALOHA Comparison of MAC protocols in wired systems and wireless systems
    • Fr ame transmission can only start at fixed times
    • Since the vulnerable period is now halved,
    Fig. : Throughput versus offered traffic for ALOHA systems
  • 11. Carrier Sense Multiple Access (CSMA) Comparison of MAC protocols in wired systems and wireless systems
    • CSMA protocols do better than ALOHA : you monitor the channel
    • before and/or during transmission
    • 1-persistent
    • Nonpersistent
    • p-Persistent
    • .
  • 12. Comparison of MAC protocols in wired systems and wireless systems Fig. : Comparison of channel utilization versus load for various random access protocols
  • 13. CSMA with Collision Detection (CSMA/CD) Comparison of MAC protocols in wired systems and wireless systems
    • Sense the channel, but immediately stop transmission when you detect a collision
    • Saves Channel Bandwidth and time
    • Ethernet works like this!!!
    Fig. : CSMA/CD can be in one of the three states: Contention, Transmission or Idle .
  • 14. Collision Free Protocols Comparison of MAC protocols in wired systems and wireless systems
    • Bit-Map protocol
    • Binary Countdown Protocol
  • 15. Limited Contention Protocols Comparison of MAC protocols in wired systems and wireless systems
    • Contention systems are good when there’s not much going on
    • Collision-free systems are good when there’s generally a lot of traffic
    • What we really want is the contention strategy during light loads , and
    • collision-free strategy during rush hours !!!
    • Solution: Limited Contention Protocols
    • Dynamically regulate the number of competing stations during a contention period.
    • If there’s not much traffic, the first station will be immediately
    • allowed to transmit a frame.
    • With a lot of traffic, the strategy reduces to the bit-map protocol.
  • 16. Limited-Contention Protocols The Adaptive Tree Walk Protocol Comparison of MAC protocols in wired systems and wireless systems
  • 17. Wavelength Division Multiple Access Protocols Comparison of MAC protocols in wired systems and wireless systems
    • If you have a lot of bandwidth, just divide the channel into sub-channels, and dynamically allocate the sub-channels !!!
        • used in fiber optics
    Fig. : Wavelength Division Multiple Access
  • 18. Ethernet Comparison of MAC protocols in wired systems and wireless systems
    • CSMA/CD based
      • sense the channel, wait until idle, and backoff if collision
    • Exponential Backoff
    • Near implementation of IEEE 802.3 protocol
  • 19. 802.3 Frame Layout Comparison of MAC protocols in wired systems and wireless systems
    • Preamble : Seven times 10101010 is used to synchronize the receiver’s clock with that of the sender
    • Start : Just a delimiter to tell that the real info is now coming
    • Address : Generally 48-bit fields. Leftmost bit indicates ordinary or group addresses (multicast / broadcast). Second bit indicates global or local address
    • Length : Ranges from 0-1500. Frames should always be at least
    • 64 bytes
    • Pad : used to fill out the frame to the minimum size
    • Checksum : Calculated over the data field. CRC-based
  • 20. Wireless Networks Comparison of MAC protocols in wired systems and wireless systems
    • Often, there are a number of base stations (a.k.a. access points) connected through guided media
    • Nodes can also group together to form an ad hoc network
  • 21. Wireless LAN Requirements Comparison of MAC protocols in wired systems and wireless systems
    • Throughput
    • Number of Nodes
    • Connection to backbone LAN
    • Service Area
    • Battery Power Consumption
    • Transmission Robustness and Security
    • Collocated Network Operation
    • License-free Operation
    • Handoff/ roaming
    • Dynamic Configuration
  • 22. Problems Comparison of MAC protocols in wired systems and wireless systems
    • There can be subtle interference:
    • Issue (a)
    • How can C be prevented from trying to transmit something to B ?
        • in that case it will ruin any receipt by B
        • hidden station problem
    • Issue (b)
    • How can we tell C that it is allowed to transmit to D, because this will not interfere with the communication from B to A ?
        • exposed station problem
  • 23. Multiple Access with Collision Avoidance (MACA) Comparison of MAC protocols in wired systems and wireless systems
    • A first sends a Request To Send (RTS)
    • B answers with a Clear To Send (CTS)
    • C hears only RTS and can freely transmit, knowing it will not interfere with A’s transmission- solves exposed station problem
    • D hears only the CTS and keeps still for otherwise it would
    • interfere with B’s reception- solves hidden station problem
  • 24. MACA for Wireless (MACAW) Comparison of MAC protocols in wired systems and wireless systems
    • G ets rid of Ethernet like unfairness associated with binary exponential backoff algorithms !!!
    • Acknowledges the importance of link layer acknowledgements
      • Makes the protocol from RTS- CTS-Data to RTS-CTS-Data-ACK
    • Significantly complex protocol
      • Performance loss with lightly loaded channel
      • Better throughput and fairer allocation in presence of high loads
  • 25. Wireless LANs MAC Sublayer Comparison of MAC protocols in wired systems and wireless systems
    • Problem : How do we solve the hidden / exposed station problem ?
    • One way or the other, stations should not be allowed to continuously interfere with each other’s transmissions
  • 26. Wireless LANs MAC Sublayer Comparison of MAC protocols in wired systems and wireless systems
    • IEEE 802.11 provides two methods to deal with this problem:
        • Distributed coordination : let the stations figure it out by using a
    • collision avoidance protocol (CSMA/CA)
        • Point coordination : there’s a central base station that controls who
    • goes first- mostly used
    • Collision detection is hard to implement on the wireless medium
        • radios cannot transmit and receive on the same frequency (half-duplex channels)
    • Solution : Use collision avoidance protocol
      • Two method like operations
        • Ethernet-like
        • MACAW
  • 27. CSMA/CA Ethernet-like Comparison of MAC protocols in wired systems and wireless systems
    • Similar to non-persistent ALOHA
        • there random time was used before re-sensing the channel when busy
        • exponential backoff is used
    • Nowadays most MAC are reprogrammable by users
        • vulnerable to cheating !!!
  • 28. Comparison of MAC protocols in wired systems and wireless systems Fig. : IEEE 802.11 Medium Access Control Logic
  • 29. CSMA/CA- MACAW Comparison of MAC protocols in wired systems and wireless systems
    • MACAW : send RTS/CTS packets to see whether you should defer
    • transmission to avoid interference with another transmission
    • Evolution of MACA
    • Network Allocation Vector (NAV): it’s a virtual channel that a station
    • assigns to itself telling it to shut up!!!
    • Note: hidden station problem is solved but NOT exposed station problem!
  • 30. CSMA/CA- point Coordination Comparison of MAC protocols in wired systems and wireless systems
    • In PCF the base-station polls the other stations, asking them if they have anything to send
    • It sends a beacon frame once every 10 or 100 ms
    • Sleep state to save battery
    • When base station transmits, there can be no hidden terminals
    • PCF and DCF can co-exist together
      • By carefully defining the inter-frame time interval
      • First the base station can poll the other stations
      • I f nobody replies, any station can acquire the channel
  • 31. Comparison of MAC protocols in wired systems and wireless systems Fig. : IEEE 802.11 Protocol Architecture
  • 32. 802.11 MAC Frame Layout Comparison of MAC protocols in wired systems and wireless systems
    • Type : Data, control, or management frame
    • Subtype : Are we dealing with RTS, CTS, an ACK, etc
    • DS : Is the frame entering/leaving the current cell?
    • MF : Frames are allowed to be fragmented to increase reliability. This bit tells whether more fragments are on their way
    • Retry : Is this a retransmission?
    • O : Stick to ordered delivery of frames
  • 33. 802.11 MAC Frame Layout Comparison of MAC protocols in wired systems and wireless systems
    • Duration : Tells how long the transmission of this frame will take, allowing other stations to set their NAV accordingly
    • Addresses : Source/destination in a cell; and those of base stations outside the cell when dealing with inter-cell traffic
    • Sequence : Sequence number of this frame. 4 bits are used to
    • identify a fragment of a frame
    Note : 802.11 supports encryption at MAC level, Ethernet does not !!!
  • 34. Broadband Wireless Comparison of MAC protocols in wired systems and wireless systems Goal : Use wireless connection between buildings i.e. avoiding the use of the local loop
    • More bandwidth is needed: 10-to-66 GHz frequency range
    • Full-duplex communication possible
    • M uch of the mobility stuff from 802.11 is not needed
    • Can be focused in directional beams ( IEEE 802.11 is omnidirectional )
    WiMAX (World Interoperability for Microwave Access) is a family of IEEE 802.16 aiming at replacing ADSL and cable modems
  • 35. Broadband Wireless Comparison of MAC protocols in wired systems and wireless systems
    • MAC : the base station controls the systems by scheduling downstream and upstream channels
        • it is connection-oriented ( QoS needed by phone-companies)
    • Time Division Duplexing (TDD)
        • Downstream traffic is mapped onto time slots by the base station
        • Upstream traffic is more complex
          • The base-station pre-allocates it ( constant bit rate service )
          • The base-station periodically polls stations ( variable bit rate service )
          • No polling but subscribers have to contend ( best-effort service )
    Fig. : TDD frame contatinig time slots
  • 36. Broadband Wireless- 802.16 Frame Structure Comparison of MAC protocols in wired systems and wireless systems Fig. : (a) a generic frame (b) a bandwidth request frame
    • Connection-oriented Service
    • Checksumming optional
    • Encryption is critical for the system
      • Managed at MAC level
  • 37. Wireless Sensor Networks Zigbee Comparison of MAC protocols in wired systems and wireless systems
    • IEEE standard 802.15.4
    • N ame of a specification for a suite of high level communication protocols using small, low-power digital radios
    • MAC layer: CSMA/CA
  • 38. Conclusion Comparison of MAC protocols in wired systems and wireless systems
  • 39. Comparison of MAC protocols in wired systems and wireless systems Fig: Channel allocation methods and systems for a common channel
  • 40. Conclusion Comparison of MAC protocols in wired systems and wireless systems
    • Simplest allocation schemes: FDM and TDM
      • Poor choices for large, variable or bursty traffic
      • Alternative: ALOHA (Dynamic Allocation)
    • Carrier Sensing in LANs and MANs led to variety of protocols
      • Binaray Countdown
        • Eliminates contention completely
      • Tree Walk
        • Reduces contention by dividing stations dynamically
    • Ethernet
      • Dominant in Wired LANs
      • Uses CSMA/CD
  • 41. Conclusion Comparison of MAC protocols in wired systems and wireless systems
    • Wireless LANs
      • CSMA does not work here
      • MACA and MACAW
      • FHSS and DSSS
      • IEEE 802.11 suggests CSMA/CA
    • Broadband Wireless
      • 802.16, TDD
    • Wireless Sensor Networks
      • Zigbee
        • CSMA/CA
  • 42. References
    • Andrew S. Tanenbaum, Computer Networks, Fourth Edition
    • William Stallings, Data and Computer Communications, Eighth Edition
    • Holger Karl, Andreas Willig, Protocols and Architectures for Wireless Sensor Networks
    • Paolo Costa, Compter Networks- Medium Access Control Sublayer
    • Alaa Muqattash, Marwan Krunz, CDMA-Based MAC Protocol for Wireless Ad Hoc Networks
    • Sushant Jain, Ratul Mahajan, Wireless LAN MAC protocols
    Comparison of MAC protocols in wired systems and wireless systems
  • 43. Thank You!!! Any Questions??? Comparison of MAC protocols in wired systems and wireless systems